1. Field of the Invention
The present invention relates to a cyclic noise-reducing apparatus for reducing noise included in a video signal.
2. Description of the Related Art
It has heretofore been known a cyclic noise-reducing apparatus of a type wherein video signals are averaged for each frame period on a time basis by paying attention to the fact that the video signals have a strong autocorrelation between frames and the component of noise included in the video signals has generally little autocorrelation, whereby the energy of the noise component is only reduced without substantially changing the energy of the signal component.
In a conventional cyclic noise-reducing apparatus, mixing means for mixing together an A/D converted input video signal and a processing signal to be mixed in a mixture ratio corresponding to a coefficient k is provided therein. An output signal of the mixing means is delayed by a period corresponding to 1 field by means of a field memory or the like and the output thus delayed is applied to the mixing means as the mixed processing signal. A video signal issued from the mixing means becomes a output signal of the apparatus. Where it is desired to write data into the field memory, odd and even fields are treated as 263H and 262H (H represents one horizontal scanning period) respectively. In addition, it has also been practiced to sub-sample digital data with a frequency of 1/2 time that of a clock signal for A/D conversion in order to reduce the capacity of the memory. Thinned video signals are subjected to interpolation by means of a suitable means as a result of the sub-sampling. However, when the interpolated data are arranged in alignment to the vertical direction, they fall into a striped pattern. It is therefore preferable to invert sampling positions or points for each prescribed horizontal scanning line.
Further, when data interpolation is effected with the data in the vertical direction, the interpolation is made with chroma signals synchronized in phase with each other. Therefore, a clock signal for the sub-sampling is inverted for each 2nH (n=1, 2, 3, . . . ).
Now, the output signal to be issued from the noise-reducing apparatus can be represented as follows: EQU A-kf(A-B)
f(A-B) shows an input/output characteristic of a non-linear circuit employed in the apparatus represented by a function.
The reference characters A (i.e., an input to the memory) and B (a signal delayed by 1 field in the memory) are both mixed together even for each data. Let's here assume that k is equal to 0.5 and the output of the non-linear circuit, i.e., f(A-B) is equal to A-B for the simplicity of illustration. At this case, the following equation is derived: EQU 0.5A+0.5B=0.5(A+B)
A and B are therefore regarded as being composed uniformly.
Let's here assume that the first field and the following second field for digitized video signals as input video signals are represented as shown in FIG. 1(a) and FIG. 1(b), respectively. In this case, the reference character A is represented as shown in FIG. 1(b), whereas the reference character B corresponds to interpolation-processed signals (which have data shown in FIG. 2) in the first field prior to 1 field. Data to be sub-sampled as the second field are as shown in FIG. 3 in which the data indicated by .largecircle. (circle) are to be sub-sampled.
Paying attention to the third line (third horizontal scanning), data having different lines represented by A.sub.n remain as a result of the sampling. As a consequence, false data are circulated, so that a satisfactory noise-reducing effect cannot be obtained.